1. Field of the Invention
This invention relates to a self-vibrating motor used for a wireless silent alerting device or a massager.
2. Description of the Prior Art
One of the prior known vibrator motors is shown in FIG. 4. In FIG. 4, a vibrator motor M has an output shaft S on which an eccentric weight W, made of copper-tungsten alloy etc., is mounted. Some danger of mechanical interference pertains to this type of vibrator motor since the rotating eccentric weight is located outside the motor case. In addition, the motor requires an extra space in which the eccentric weight turns.
In Japanese laid-open patent application, application No. 2-294482 (application date Oct. 31, 1990), which has been assigned to the assignee of the present invention, discloses a vibrator motor in which one of ordinarily three armature poles is deleted and the rotor has two armature poles angularly spaced at 120.degree. each other. However, this type of vibrator motor has a large cogging effect and, therefore, requires comparatively high voltage to obtain a large power for starting the rotation in order to overcome the cogging effect. Such vibrator motor is less suitable for a battery-operated pocket type alerting device.
U.S. Pat. No. 5,036,239 (issued Jul. 30, 1991) assigned to the assignee of the present invention discloses a miniature flat vibrator motor employing a flat eccentric rotor which is rotatably supported by a pair of bearings.
In the case of a miniature flat motor having a very small thickness, the coaxial alignment tolerance between the two bearings must be made extremely small in order to maintain a proper clearance between the rotor and the housing. Obtaining such a small positional tolerance of the bearings, extremely small machining tolerances of the components are required.